Evaluation of Compressive Strength of Several Pulp Capping Materials

Statement of the Problem: Adequate compressive strength is an important characteristic for an ideal liner. Purpose: This study aimed to assess the compressive strength of several commonly used liners. Materials and Method: This in vitro, experimental study evaluated 120 samples fabricated of Dycal, Calcimol LC, Vitrebond, Activa Bioactive, and TheraCal LC (n=24) liners according to the manufacturers’ instructions. The samples were fabricated using a cylindrical stainless steel mold with 6±0.1 mm height and 4±0.1 mm internal diameter. Half of the samples in each group (n=12) underwent compressive strength test immediately after completion of their primary setting while the other half (n=12) underwent compressive strength test after 24 h. During this period, the samples were immersed in deionized water (grade 3) and incubated at 37±1°C and 100% humidity for 24 h. The compressive strength was measured using a universal testing machine. Data were analyzed using two-way ANOVA followed by Tukey’s post-hoc test. Results: The compressive strength of the five liners was significantly different (p< 0.05). Calcimol LC showed maximum compressive strength both immediately after setting and after 24 h. The compressive strength at 24 h was significantly higher than the primary compressive strength in all groups (p< 0.05). Conclusion: Within the limitations of this study, it seems that Calcimol LC, Activa Bioactive Liner, and TheraCal LC have adequate compressive strength and can be used alone to provide adequate support for the restorative materials.


Introduction
Restoration of teeth and preservation of pulp vitality are the main goals of restorative dental treatments [1]. Pulp exposure may occur due to deep carious lesions or mechanical trauma (such as iatrogenic trauma during tooth preparation), leading to pulp infection and pain. Root canal treatment is relatively invasive, time and cost consuming. Alternatively, vital pulp therapy such as pulp capping may be indicated for such cases [2][3][4]. Pulp capping treatment is suitable for teeth with no clinical sign/symptom of irreversible pulpitis and necrosis, and no radiographic sign of periapical involvement [5][6].
The success rate of pulp capping treatment ranges from 72.9% to 95.4% [7]. Pulp capping treatment aims to protect the pulp against physical, chemical, thermal and electrical stimuli (such as galvanic effect of amalgam restorations) as well as microorganisms. It aims to preserve the pulp vitality, seal the dentinal tubules and induce the formation of dentinal bridge by odontoblasts and pulp cells (as the ultimate goal of pulp capping treatment) [8][9][10]. Pulp capping treatment requires the application of one or more layers of pulp capping agent.
Base materials are applied beneath the restorative material when the thickness of the remaining dentin is more than 0.5 mm, and liners are indicated when the thickness of the remaining dentin is less than 0.5mm [10][11][12].
Pulp capping agents are in close contact with the pulp tissue and thus, should be non-toxic and biocompatible. They should be able to provide an optimal seal, minimize microleakage, release fluoride and bond to dentin and restorative materials. They should have low solubility, optimal bio-interactivity and bioactivity, dimensional stability, bactericidal or bacteriostatic property, radiopacity, and adequately high compressive strength [11,[13][14]. None of the available pulp capping agents have all the above-mentioned properties; thus, their selection depends on the opinion of dental clinician and clinical conditions [10,15].
Calcium hydroxide (CH) has long been used as a pulp capping agent due to its excellent antimicrobial property, induction of formation of dentinal bridge, low toxicity and high clinical success rate. It was the goldstandard pulp capping agent for several decades [9,[16][17]. However, low compressive strength is a major drawback of CH. Thus, it requires the application of glass ionomer (GI) or zinc oxide eugenol to provide adequate compressive strength beneath the restorative materials [8,18]. Calcimol LC is a new CH-based liner with a photo-initiator. It was introduced to the market to eliminate the shortcomings of conventional, autopolymerizing CH such as Dycal [19].
GIs are also among the commonly used dental materials with advantages such as the ability to absorb and release fluoride and bonding to enamel and dentin.
However, moisture sensitivity and low compressive strength are among their drawbacks [20][21]. Some modifications were made in the composition of GIs, which resulted in introduction of resin-modified GIs. Vitrebond is a commercially available resin-modified GI, which is used as a liner in pulp capping treatment of teeth [22]. Further modifications in the composition of GI powders resulted in advent of GIs containing bioactive glass. Resin matrix was also added to light-cure GIs for further improvement in their mechanical properties, yielding products such as Activa Bioactive base/liner [23][24].
Later on, mineral trioxide aggregate (MTA) was introduced to the market with optimal properties such as biocompatibility, induction of dentinal bridge formation, antimicrobial properties, high pH, and radiopacity. Due to drawbacks such as long setting time, and poor mechanical properties such as low compressive strength and difficult handling, Biodentine, was introduced as alternative calcium silicate-based cement to MTA and as a replacement for dentin [22]. Biodentine is biocompatible, induces dentinal bridge formation, has antimicrobial properties, provides a better sealing than CH, and has a shorter working time and easier handling compared with MTA [25]. TheraCal LC is a new liner from the family of calcium silicate cements modified with light-cure resin [22].

Materials and Method
This in vitro, experimental study measured the compressive strength of 120 samples fabricated from five liners according to ISO 9917-1,2 (2007) for dental cements [29]. Sample size was calculated to be 24 samples in each of the five cement groups according to a previous study [27], assuming alpha=0.05, beta=0.  Table 1 presents  Single paste was injected into the mold in 6 increments each with 1 mm thickness using a special syringe. Each increment was light-cured for 20 s.

Calcimol LC (VOCO GmbH, Cuxhaven, Germany)
Single paste was injected into the mold in 6 increments each with 1 mm thickness using a special syringe. Each increment was light-cured for 20 s.

Statistical analysis
Data were analyzed using SPSS version 25 (SPSS Inc., IL, USA). Two-way ANOVA was applied to assess the effect of time and type of liner on compressive strength.
Pairwise comparisons of the groups were carried out using the Tukey's HSD post-hoc test. p≤0.05 was considered statistically significant. Table 2    of dentin or the permanent restorative material applied over them [31][32]. According to Douglas [33], compressive strength is the best quality control measure that can be considered by the manufacturers to produce a high-quality restorative material. 2.5% to 5% of this liner [40]. This resin-rich network is responsible for maximum compressive strength of this liner [19] superior to that of TheraCal LC. Higher solubility of Dycal also explains its lower compressive strength than Calcimol LC and TheraCal LC [19]. The same result was reported by Nielsen et al. [28]. They also showed that both TheraCal LC and Dycal had higher compressive strength at 24 h compared with 15 min, which was in line with our findings.

Results
Mitra [41] showed higher compressive strength of Vitrebond than conventional GI at 1 and 24 h. Also, the compressive strength of Vitrebond at 24 h was higher than that at 1 h. The setting reaction of GI is a gradual process that may take up to 1 month. Eliades et al. [42] showed that the compressive strength of Vitrebond at 1 month was higher than that at 24 h. Increase in compressive strength of GI over time was also noted in our study. Increase in strength of GIs over time can be due to the slow formation of silica matrix during polymerization. Also, it has been demonstrated that strength of GIs depends on gradual degradation of poly-acrylic acid copolymers [43].
In our study, Activa Bioactive liner ranked second in terms of compressive strength while Yli-Urpo et al. [27] showed that the compressive strength of GIs (conventional and resin-modified types) decreased following the addition of bioactive glass due to their weak bonds to GI matrix during the mixing phase. However, Activa Bioactive liner does not have this separate mixing phase.
Activa Bioactive liner has a triple polymerization mechanism including light-cure resin setting, self-cure resin polymerization and self-cure acid-based chemical reactions of GI [44]. Also, the resin matrix added to it has a double-cure polymerization mechanism, which explains its superior mechanical properties compared with Vitrebond [24]. Activa Bioactive liner does not contain bi-